1. Field of the Invention
The present invention relates to gamut mapping by which a color gamut of an input device is converted into a color gamut of an output device via a device-independent standard color space.
2. Description of the Related Art
In recent years, a digital device such as a digital camera and an image scanner has been widespread, and thus a digital image can readily be obtained.
In addition, a technology for producing a full color hard copy has shown rapid progress. For example, the image quality of ink jet system printing has become as good as silver halide photography, and thus the ink jet system printing has been widely used.
Meanwhile, a network such as the Internet has become widespread, and thus, a great many users can connect various devices to the network.
Under an environment in which various types of input/output devices are connected, color image data is input and output between devices with different color reproduction ranges (color gamuts).
For example, color image signals having a certain color reproduction range that are displayed by the monitor are very frequently printed in a hard copy by using a printer having a narrower color reproduction range.
On the other hand, for a technology for color reproduction of the same color between different devices, a “color management system” (hereinafter simply referred to as “CMS”) is known.
FIG. 1 is a diagram illustrating an overview of an exemplary configuration of the CMS. FIG. 1 illustrates the CMS that uses a device-independent color space.
That is, FIG. 1 illustrates a case where an image input device such as a camera or a scanner and an image output device such as a printer or a monitor are mutually connected, for example.
As shown in FIG. 1, a processing of converting a color signal that is processed by a certain input device into a color signal for an output device is implemented via a device-independent color space (for example, CIE-XYZ and CIE-L*a*b*) by using profiles for the input device and the output device.
Here, a “profile” is a description of a conversion formula that connects each device-dependent color with the device-independent color space or a description of a conversion table that is previously produced as a lookup table of the conversion formula.
For example, in an ICC profile, the CIE-XYZ color space and the CIE-L*a*b* color space under a D50 light source are defined as the device-independent color space used in the CMS (hereinafter referred to as a “standard color space”).
Note here that the “ICC” refers to the “International Color Consortium”.
In addition, under an environment where Windows®, which is an operating system of Microsoft Corporation, is used, another standard color space, namely, an “sRGB” color space (IEC 61966-2-1) is used.
In addition, recently, an Adobe RGB color space that is advocated by Adobe Systems, Inc. is used as the standard color space, in the input device such as a high-end digital camera.
The device-independent color reproduction can be implemented via the standard color space that is the device-independent color space.
In reproducing the color in the CMS by each device, in order to reproduce the color by the output device that can be reproduced by a certain device, a gamut mapping technology is used. In the gamut mapping, a difference in the color reproduction range between the input device and the output device is accommodated.
Japanese Patent Application Laid-Open No. 06-225130 discusses a mapping of image data to a color gamut of the device whose color reproduction range is narrower.
In addition, Japanese Patent Application Laid-Open No. 06-225130 discusses a method in which the color outside the color reproduction range of the device is compressed to an outermost shell of the color reproduction range of the device. In this method, a saturation only is subjected to the compression, while maintaining a lightness and a hue at a constant level.
There is another method of the gamut mapping. Japanese Patent Application Laid-Open No. 06-225130 discusses the gamut mapping such that the outside the color reproduction range of the device is compressed to the color so that a color difference in the CIE-L*a*b* color space is minimized.
However, the method by which the gamut mapping performed via the standard color space is implemented by using the lookup table (hereinafter referred to as an “LUT”) has drawbacks as described below.
More specifically, there is a drawback arising with respect to a relationship between a size and shape of the standard color space and the color reproduction range of the input device and the output device.
In this regard, first, there is a drawback caused with respect to an inclusion relationship between the size of the standard color space and the color reproduction range of each of the input device and the output device.
FIG. 2 is a diagram illustrating a case where in producing the LUT, which is the profile for the output device, the standard color space does not include the whole part of the color reproduction range of the output device.
FIG. 2 illustrates an sRGB color space 201 as the standard color space and a color gamut 202 of an inkjet printer, which is an example of the output device, in the CIE-L*a*b* color system.
In addition, a shaded region 203 indicates a range of the printer color gamut that extends off the standard color space. If the color outside the color gamut is input from the input device, information at the region 203 is lost at the time of conversion into the standard color space.
That is, in order to effectively use the whole color gamut of the output device and exploit a full color reproduction capacity of the output device, it is necessary to use the standard color space that includes the color gamut of the output device.
On the other hand, there is a case where the standard color space that satisfies the condition is far larger than the color gamut to which the color is estimated to be actually input (hereinafter referred to as an “input-estimated color gamut”), and, therefore, there is a drawback caused by this.
The standard color space tends to be wider than the input-estimated color gamut because of the following reasons. The first reason is that the standard color space in the specification of the ICC profile is used. The second reason is that with respect to the output device having any color reproduction range, the LUT is produced after the standard color space is defined so that the whole color reproduction range is included in the standard color space.
Here, the input-estimated color gamut is the color gamut as described below. For example, the description is made as to a case where the output device is an inkjet printer and an input image to be printed is a photographic image that is photographed by a digital camera.
Generally, in a popular type digital camera, a color signal obtained by an input sensor is converted into YCbCr data by a given processing, estimating the compression to the sRGB color space.
That is, the color input through the digital camera is turned into the color within the sRGB color space. In this case, the sRGB color space is defined as the input-estimated color gamut.
In this manner, in producing the LUT, the color gamut of the input that is input most in a system that uses the LUT is defined to be the input-estimated color gamut.
When the input-estimated color gamut is the sRGB color space as described above and the standard color space is the CIE-L*a*b* color space or the Adobe RGB color space, for example, the standard color space is wider than the sRGB color space which is the input-estimated color gamut.
FIG. 3 is a diagram illustrating a relationship among a CIE-L*a*b* color space 301, a printer color gamut 302, and an input-estimated color gamut (sRGB color space) 303. Here, the CIE-L*a*b* color space 301 is an example of a wide standard color space.
In converting the CIE-L*a*b* color space 301 by using the profile for the output device and performing the gamut mapping to the printer color gamut 302, a color X that is a color positioned in the color gamut outside the input-estimated color gamut 303 also needs to be compressed to the color gamut of the output device.
That is, the color that is not likely to be actually input needs to be compressed with a secured color reproduction by the output device.
Thus, the color gamut within the printer color gamut 302 is allocated even for the color that is not likely to be actually input, and accordingly, the region in which the color of the input-estimated color gamut 303 is reproduced is small.
In this case, when a photograph is printed by the printer, the quality of a resulting photographic image is not good because a color collapse occurs and the saturation is degraded.
A second drawback is caused because the size of the color gamut of the output device compared to the standard color space is different depending on a print medium that is used.
Generally, the size of the color gamut of the output device differs depending on the type of the print medium and a targeted print quality. In this regard, when the output device is an inkjet printer, the color gamut in the case of printing onto a photograph paper is wide and the color gamut in the case of printing onto a plain paper is narrow.
Accordingly, when the standard color space that includes the color gamut of the output device is used, a color reproducibility in the case where the mapping is performed in a manner such that the color gamut of the photograph paper providing a gradation differs from the color reproducibility in the case where the mapping is performed with respect to the color gamut of the plain paper.
That is, in mapping to the color gamut of the photograph paper, the saturation and the lightness are not degraded much, and in mapping to the color gamut of the plain paper, the saturation and the lightness are degraded very much.
As a result, the print image gets somber and a contrast becomes low. Thus, a good reproduction of color cannot be implemented.